Whereas strong antioxidant properties of spermine have been reported mostly in in vitro studies, there is lack of the in vivo studies on spermine influence conducted on mammals. The main objective of this study was to investigate the effects of different doses of spermine and the period of its supplementation on the liver and spleen antioxidant capacity in weaned rats. Male Sprague-Dawley rats at the age of 19 days received intragastrically spermine at the dose of 0.2 or 0.4 μmol · g^-1 body weight for 3 or 7 days, respectively. Control rats received saline in analogical way. It was found that liver anti-superoxide anion (ASA) capacity, catalase (CAT) activity, glutathione (GSH) content and total antioxidant capacity (T-AOC) were increased in group supplemented with higher dose of spermine after 3 days, and anti-hydroxy radical (AHR) capacity was increased when treatment lasted for 7 days. In the spleen the higher spermine dose supplementation increased ASA capacity and total superoxide dismutase (T-SOD) activity (after 3 and 7 days), AHR capacity (after 7 days) and T-AOC (after 3 days) in comparison to the corresponding control groups (P < 0.05). Only in the spleen the lower spermine dose reduced lipid peroxidation level and increased CAT activity and GSH content regardless treatment duration (P < 0.05). The obtained results suggest that spermine supplementation can improve the antioxidant properties of the liver and spleen of weaned rats in a dose-, time- and tissue-dependent manner.